Comment by grunder_advice

Could be a lot of things and probably a bit of everything. You might have social anxiety, even if you don't feel particularly shy. If you're too clammed up, people recognize that and keep a certain distance. It could also be that you have a very rigid personality. It might simply be a matter of lack of socialization. I don't think there is any way to diagnose this and pinpoint the problem. Learning how to be social is very much a lot like learning how to program: try lots of things, spend a lot of time doing it, and don't be afraid of breaking things. Eventually you'll develop your own methods to keep people entertained and then people will take more interest in you.

Been there, done that.

BTW, you're 19 and actually at 19 this is not all that abnormal. Lots of kids in college find themselves in your situation.